8.2 Electron transport chain and oxidative phosphorylation

The electron transport chain and oxidative phosphorylation are the final stages of cellular respiration. These processes harness energy from electrons to create a proton gradient, ultimately producing ATP, the cell's energy currency.

In this section, we'll explore the complexes involved in electron transport, key electron carriers, and how the proton gradient drives ATP synthesis. Understanding these processes is crucial for grasping how cells efficiently generate energy from nutrients.

Electron Transport Chain Complexes

Complex Structure and Function

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• Complex I (NADH dehydrogenase) oxidizes NADH, transferring electrons to ubiquinone and pumping protons across the inner mitochondrial membrane
• Complex II (Succinate dehydrogenase) oxidizes succinate to fumarate, reducing ubiquinone and does not transport protons
• Complex III (Cytochrome bc1 complex) transfers electrons from ubiquinol to cytochrome c, pumping protons across the inner mitochondrial membrane (Q cycle)
• Complex IV (Cytochrome c oxidase) transfers electrons from cytochrome c to oxygen, the final electron acceptor, pumping protons across the inner mitochondrial membrane

Electron Flow and Energy Release

• Electrons flow through the complexes in order of increasing reduction potential, releasing energy at each step
• Energy released from electron transfer is used to pump protons across the inner mitochondrial membrane, generating a proton gradient
• Electron transport chain is the major site of ATP production in cellular respiration (oxidative phosphorylation)
• Inhibitors of electron transport chain complexes (rotenone, antimycin A, cyanide) can disrupt ATP production and lead to cell death

Electron Carriers

Coenzyme Q (Ubiquinone)

• Lipid-soluble electron carrier that shuttles electrons between Complex I, II, and III
• Exists in oxidized form (ubiquinone) and reduced form (ubiquinol)
• Accepts electrons from NADH (via Complex I) and FADH2 (via Complex II), becoming reduced to ubiquinol
• Donates electrons to Complex III, becoming oxidized back to ubiquinone

Cytochrome c

• Water-soluble electron carrier that shuttles electrons from Complex III to Complex IV
• Heme-containing protein that alternates between reduced (ferrous, Fe2+) and oxidized (ferric, Fe3+) states
• Accepts electrons from Complex III (cytochrome c1) and donates them to Complex IV
• Cytochrome c release from mitochondria can trigger apoptosis (programmed cell death)

Proton Gradient and Membrane

Proton Gradient Formation and Function

• Proton gradient is formed by the pumping of protons (H+) from the mitochondrial matrix to the intermembrane space
• Complexes I, III, and IV contribute to the proton gradient by coupling electron transfer to proton pumping
• Proton gradient is used to drive ATP synthesis through ATP synthase (chemiosmotic coupling)
• Proton gradient also powers other processes (mitochondrial protein import, metabolite transport)

Redox Reactions and Electron Transport

• Redox reactions involve the transfer of electrons between molecules, with one molecule being oxidized (losing electrons) and the other reduced (gaining electrons)
• Electron transport chain involves a series of redox reactions, with electrons being transferred from NADH and FADH2 to oxygen
• Redox potential difference between electron donors (NADH, FADH2) and acceptors (ubiquinone, cytochrome c, oxygen) drives electron flow
• Electron transport is coupled to proton pumping, converting redox energy into a proton gradient

Inner Mitochondrial Membrane Structure and Function

• Inner mitochondrial membrane is highly folded, forming cristae that increase surface area for electron transport and ATP synthesis
• Electron transport chain complexes and ATP synthase are embedded in the inner mitochondrial membrane
• Inner mitochondrial membrane is impermeable to protons, allowing the formation and maintenance of the proton gradient
• Cardiolipin, a unique phospholipid found in the inner mitochondrial membrane, is essential for the function of electron transport chain complexes and ATP synthase